Experimental validation of some basic assumptions used in physically based soil

In spring 2009, four rill experiments were accomplished on a fallow land. Most external factors as well as discharge quantity (9 L min-1) were held constant or at least in the same range. Following most process based soil erosion models, detachment or runoff values should therefore be similar, but the experimental results show clear differences in sediment concentration, runoff and other measured and calculated values. This fact underlines the problems of process based models: concerning rill erosion, different processes take part and the process described by the models is only responsible for a part of the eroded material

Applicability of Different Hydraulic Parameters to Describe Soil Detachment in Eroding Rills

This study presents the comparison of experimental results with assumptions used in numerical models. The aim of the field experiments is to test the linear relationship between different hydraulic parameters and soil detachment. For example correlations between shear stress, unit length shear force, stream power, unit stream power and effective stream power and the detachment rate does not reveal a single parameter which consistently displays the best correlation. More importantly, the best fit does not only vary from one experiment to another, but even between distinct measurement points. Different processes in rill erosion are responsible for the changing correlations. However, not all these procedures are considered in soil erosion models. Hence, hydraulic parameters alone are not sufficient to predict detachment rates. They predict the fluvial incising in the rill's bottom, but the main sediment sources are not considered sufficiently in its equations. The results of this study show that there is still a lack of understanding of the physical processes underlying soil erosion. Exerted forces, soil stability and its expression, the abstraction of the detachment and transport processes in shallow flowing water remain still subject of unclear description and dependence

Measuring fast-temporal sediment fluxes with an analogue acoustic sensor: a wind tunnel study

In aeolian research, field measurements are important for studying complex wind-driven processes for land management evaluation and model validation. Consequently, there have been many devices developed, tested, and applied to investigate a range of aeolian-based phenomena. However, determining the most effective application and data analysis techniques is widely debated in the literature. Here we investigate the effectiveness of two different sediment traps (the BEST trap and the MWAC catcher) in measuring vertical sediment flux. The study was performed in a wind tunnel with sediment fluxes characterized using saltiphones. Contrary to most studies, we used the analogue output of five saltiphones mounted on top of each other to determine the total kinetic energy, which was then used to calculate aeolian sediment budgets. Absolute sediment losses during the experiments were determined using a balance located beneath the test tray. Test runs were conducted with different sand sizes and at different wind speeds. The efficiency of the two traps did not vary with the wind speed or sediment size but was affected by both the experimental setup (position of the lowest trap above the surface and number of traps in the saltation layer) and the technique used to calculate the sediment flux. Despite this, good agreement was found between sediment losses calculated from the saltiphone and those measured using the balance. The results of this study provide a framework for measuring sediment fluxes at small time resolution (seconds to milliseconds) in the field.In aeolian research, field measurements are important for studying complex wind-driven processes for land management evaluation and model validation. Consequently, there have been many devices developed, tested, and applied to investigate a range of aeolian-based phenomena. However, determining the most effective application and data analysis techniques is widely debated in the literature. Here we investigate the effectiveness of two different sediment traps (the BEST trap and the MWAC catcher) in measuring vertical sediment flux. The study was performed in a wind tunnel with sediment fluxes characterized using saltiphones. Contrary to most studies, we used the analogue output of five saltiphones mounted on top of each other to determine the total kinetic energy, which was then used to calculate aeolian sediment budgets. Absolute sediment losses during the experiments were determined using a balance located beneath the test tray. Test runs were conducted with different sand sizes and at different wind speeds. The efficiency of the two traps did not vary with the wind speed or sediment size but was affected by both the experimental setup (position of the lowest trap above the surface and number of traps in the saltation layer) and the technique used to calculate the sediment flux. Despite this, good agreement was found between sediment losses calculated from the saltiphone and those measured using the balance. The results of this study provide a framework for measuring sediment fluxes at small time resolution (seconds to milliseconds) in the field

Short-lived Nuclei in the Early Solar System: Possible AGB Sources

(Abridged) We review abundances of short-lived nuclides in the early solar system (ESS) and the methods used to determine them. We compare them to the inventory for a uniform galactic production model. Within a factor of two, observed abundances of several isotopes are compatible with this model. I-129 is an exception, with an ESS inventory much lower than expected. The isotopes Pd-107, Fe-60, Ca-41, Cl-36, Al-26, and Be-10 require late addition to the solar nebula. Be-10 is the product of particle irradiation of the solar system as probably is Cl-36. Late injection by a supernova (SN) cannot be responsible for most short-lived nuclei without excessively producing Mn-53; it can be the source of Mn-53 and maybe Fe-60. If a late SN is responsible for these two nuclei, it still cannot make Pd-107 and other isotopes. We emphasize an AGB star as a source of nuclei, including Fe-60 and explore this possibility with new stellar models. A dilution factor of about 4e-3 gives reasonable amounts of many nuclei. We discuss the role of irradiation for Al-26, Cl-36 and Ca-41. Conflict between scenarios is emphasized as well as the absence of a global interpretation for the existing data. Abundances of actinides indicate a quiescent interval of about 1e8 years for actinide group production in order to explain the data on Pu-244 and new bounds on Cm-247. This interval is not compatible with Hf-182 data, so a separate type of r-process is needed for at least the actinides, distinct from the two types previously identified. The apparent coincidence of the I-129 and trans-actinide time scales suggests that the last actinide contribution was from an r-process that produced actinides without fission recycling so that the yields at Ba and below were governed by fission.Comment: 92 pages, 14 figure files, in press at Nuclear Physics

The rill experiment as a method to approach a quantification of rill erosion process activity

Within this paper a standardized method to quantify sediment transport and runoff in natural rills is described. In order to achieve this, several rill experiments (RE) were accomplished in March 2007 in the Arnás catchment in the Spanish Pyrenees. Both, anthropogenically initiated and naturally developed rills were flushed with a total water quantity of 72 l in 8 minutes (equivalent to 9 l min-1). For the characterisation of the rill, slope is measured and micromorphological features like scours are registered. The experiments are characterised by the flow velocities along the whole flushed rill, sediment concentrations at different points and different times during the experiment. Runoff is measured after 25 m continuously. With this data, a set of characteristic variables is generated, which reflects the infiltration and flow behaviour along the rill. By means of rainfall simulations within the rills catchments, their contributing runoff was estimated also. The tested rills were developed on average slopes oscillating between 7.6° and 11.3°, the steepest slope reached 16°. The sediment concentrations reached average values between 0.69 and 2.21 g l-1, the maximum values ranged between 1.59 and 6.31 g l-1. Comparing the sediment concentrations measured in the rills to the sediment concentrations in the runoff of the river Arnás, it can be stated that the concentrations in the rills are usually higher. Though, the runoff was to low to cause erosion. Accordingly, the runoff amount that can be produced within the rills catchments was found to be about 10-25 times higher. By means of the developed rill experiments, for which easy to handle devices were built and are described in detail, it becomes possible to assess the effectivity of individual rills in a catchment and to evaluate their hydraulic functioning as well as their geomorphodynamic activit

Spatial pattern and temporal variability of runoff processes in Mediterranean Mountain environments - a case study of the Central Spanish Pyrenees

Mediterranean mountain environments like the Central Spanish Pyrenees show a highly variable rainfall-runoff response, mainly explained by the intense intra- and inter-annual variability of precipitation yield. This leads to a highly differentiated moisture status and therefore it is assumed to lead also to highly variable runoff contributing areas. For the identification of areas with certain dominant runoff processes in an experimental headwater catchment in which agriculture was abandoned several decades ago the concept of the topographical index was extended by means of weighting grids. These weighting rasters were generated using additional information on soils and vegetation. Runoff generating areas were identified widespread in the catchment, with Hortonian overland flow (HOF) dominating the runoff processes on degraded soils, and saturation overland flow (SOF) dominating the footslope areas, where hydromorphic soils were mapped. Rainfall-runoff experiments were performed to quantify runoff and erosion and to identify seasonal changes using experimental data gained in different seasons of the year. The seasonal changes in runoff response could be localised clearly within the areas of SOF, whereas the other ones showed a similar behaviour. This implied that the procedure of delineation had to be differentiated for dry and moist conditions, and that the SOF areas had to be reclassified as SSF/DP areas for dry conditions. Due to the location of these areas close to the ravine, we could explain the pronounced switching runoff behaviour of the catchment. GIS techniques combining different levels of topographic, soil and vegetation information showed to be suitable for delineation of areas with different runoff generation processes. The inclusion of seasonally distributed experimental data demonstrated that for dry conditions, slightly different methods have to be applied. Nevertheless, the study showed also the limitations of the combined methods: (I) subsurface flows and ground water recharge could only be deduced, not demonstrated, (II) finally, there is still a good knowledge of the area needed for an accurate process representation

Identification of gully-development processes in semi-arid NE-Spain

Gully erosion is considered to be one of the most important soil erosion processes, but its contribution to soil loss is still discussed. Despite the efforts made to understand the occurrence and dynamics of gullies, there is still a lack of knowledge on the factors leading to gully formation and growth. Within this study, the results from monitoring of gully-growth with high-resolution aerial photography are combined with the characterisation of the gully catchments with mapping of soil surface types and morphology and rainfall simulations. The results show an extremely high variability of gully-growth dynamics, being it independent from the size of the contributing catchment and rainfall characteristics. The distribution of soil surface types and their infiltration and erosion characteristics are identified as determinant for gully-growth. Additionally, their position related to the active gully-headcut is identified as determinant for the observed gully developmen

Field experiments for understanding and quantification of rill erosion processes

Despite many efforts over the last decades to understand rill erosion processes, they remain unclear. This paper presents the results of rill experiments accomplished in Andalusia in September 2008 using a novel experimental set up. 72 L of water are introduced with an intensity of 9 L min(-1) into a rill. Rill cross sections, slope values, flow velocities and sediment concentrations were measured and these values were used to calculate sediment detachment and transport. Each experiment was repeated once within 15 min. With this new experimental setup it is possible to calculate several hydraulic parameters like hydraulic radius, wetted perimeter, flow cross section, transport rate and transport capacity which are usually estimated from coarse flow and rill parameters. In rill experiments, four different natural rills were flooded with the same experimental setup. Several processes like transport of loose material, erosion, bank failure and knickpoint retreat and the runoff effectiveness showed different and variable intensities. The sediment concentrations ranged between 5.2 and 438 g L-1. most cases, detachment rates are close to the transport capacity and, in some cases, the transport capacity is even exceeded. This can be explained by the occurrence of different erosion processes within a rill (e.g. detachment, bank failure, and headcut retreat) which are not all explained by the given equations. The results suggest that the existing soil erosion equations based on shear forces exerted by the flowing water are not able to describe rill erosion processes satisfactory. Too many different processes with a high spatial and temporal variability are responsible for rill development. (C) 2010 Elsevier B.V. All rights reserved